[electrum] fix fee unit in Preferences > Fee > Custom Fee Rate

Summary:
Setting the XECSatsByteEdit._base_unit attribute had no effect in updating the widget's displayed unit. There is a AmountEdit.set_base_unit method for this, use it.

Remove also the pointless instance attribute in XECAmountEdit

Test Plan: start Electrum ABC, go to the preferences menu, fee tab, and check that the correct unit is displayed in the line edit idget (previously it showed "XEC")

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18330